1. Field of the Invention
The present invention relates to a variator disk for a toroidal variator and to a variator of this type which is suitable for a motor vehicle toroidal transmission.
2. Description of the Related Art
In the field of transmissions for motor vehicles, there is a trend toward continuously variable transmissions. Continuously variable transmissions make it possible, in general, to operate the series connected internal combustion engine in motor vehicles within a favorable rotational speed range independently of the respective motor vehicle speed. This improves the efficiency of the drive train formed by the internal combustion engine and by the continuously variable transmission. Furthermore, continuously variable transmissions afford a particularly high degree of driving comfort.
Among continuously variable transmissions, toroidal transmissions, as they are known, are especially important, specifically, in particular, owing to their higher torque capacity, as compared with belt type continuously variable transmissions (CVTs).
Among toroidal transmissions, the Torotrak™ system has particular significance (cf. www.torotrak.com).
A typical toroidal transmission of this type has a variator arrangement with two variators. Each variator has two variator disks. The disks have annular traction or running surfaces facing one another which in each case define a toroidal space. The toroidal spaces are arranged coaxially with respect to a variator axis. Within the toroidal spaces, rollers are in each case arranged, which come into engagement with the variator disks in order to transfer a torque from one variator disk to the other variator disk. The rollers are arranged so as to be distributed over the circumference of the toroidal space and can be adjusted spatially within the toroidal space, in order to adjust the transmission ratio of the variator continuously.
In the prior art, the variator disks are connected to an assigned variator shaft, as a rule via toothings in the hub region.
Furthermore the variator disks known hitherto are supported axially on the variator shaft via a collar. This collar is small in comparison with the diameter of the variator disks. This gives rise to a high bending moment. High stresses are generated within the component as a result of the bending moment. These can be absorbed only by means of a high-mass type of construction.
The high-mass type of construction of the variator disks leads to a high weight, to a high mass moment of inertia and to an enlargement of the overall construction length of the transmission. The high mass moment of inertia reduces the dynamics of the vehicle. Due to the high weight, high material costs are incurred, since the disks are produced as a rule, from high-quality material.
The high axial pressure forces, particularly also during an adjustment of the rollers, result in a high load on the variator disks in the vicinity of the toothing. This may lead to excessive stresses and consequently cause failure.
The manufacture of the toothings is cost-intensive, particularly since the material of the known variator disks is, as a rule, a high-strength rolling bearing steel.
So that the high axial forces can be absorbed more effectively, one variator disk may be assigned a supporting disk which is arranged on that side of the variator disk which lies opposite the running surface. By virtue of the supporting disk, it is possible to produce the variator disk with a lower mass and lower weight. Costs are thereby saved. In other words, in this embodiment, the variator disk is formed by at least two partial disks which are designed as a traction disk, on which the running surface is formed, and as a supporting disk, respectively. The supporting disk is designed for supporting axial forces applied to the running surface.
Furthermore, as a rule, the supporting disk is connected to the shaft positively in the circumferential direction. This may take place via a toothing. However, the supporting disk may also be produced in one piece with the shaft.
Although it is conceivable, even when a supporting disk is used, to connect the traction disk to the shaft via a toothing in the hub region, it is nevertheless preferable to cause the transfer of torque from the traction disk to the shaft to take place via the supporting disk.
This may take place in general by means of nonpositive or frictional connection. This is because the high axial pressure forces can be utilized for torque transfer if the axial bearing surfaces on the supporting disk, on the one hand, and on the traction disk, on the other hand, are suitably designed. In this case, the coefficient of friction of steel/steel is utilized in the bearing region.
However, the transfer of the torque from the traction disk to the supporting disk may also take place positively. In this case, it is conceivable, in general, to connect the traction disk positively in the hub region of the supporting disk. In order to reduce the circumferential forces, however, it is more beneficial to implement the torque transfer in the region of the outer circumference of the supporting disk or of the traction disk.
In this case, it is likewise conceivable, in general, to provide a toothing in the circumferential region. It is considered more favorable, however, to insert positive elements, such as, for example, balls, into corresponding radial recesses of the supporting disk, on the one hand, and of the traction disk, on the other hand.
These approaches have in common the fact that excessive stresses due to the notch effect may occur in the region of positive connection of the traction disk and supporting disk.
The object of the present invention is to specify an improved variator disk, in particular a variator disk which possesses a low weight and requires a small construction space.